1. Field of the Invention
This invention relates to an interferometric, self-homodyne optical receiver and method for the detection of digitally phase modulated optical signals without requiring a local oscillator.
2. Description of the Relevant Art
There are many different forms of optical detectors. A popular form of detector employs coherent optical techniques, including optical heterodyne/homodyne detection.
In both homodyne and heterodyne receivers, a local laser oscillator is used to provide a reference signal which is then mixed with the incoming optical signal to produce a difference or intermediate frequency, generally at radio frequency (RF). The RF intermediate frequency can then be processed using relatively straightforward RF techniques. Thus, receivers operating according to the heterodyne principle use a local laser to mix the received signal and, consequently, generate an RF signal. If the local laser and the received optical signal have identical frequencies or wavelengths, then the receiver will operate according to the so-called homodyne principle. Thus, homodyne-type receivers operate much like heterodyne receivers having an intermediate frequency centered about zero Hertz.
Homodyne-type receivers require a local oscillator which can either be phase- or frequency-locked to the input signal carrier such that the local oscillator can track a wide range of frequency and phase variations within the incoming signal. The local oscillator signal can be set precisely at the frequency of the incoming signal either in phase or 180.degree. out of phase with the signal to provide necessary phase locking. An error signal generated from a differential amplifier is used to provide phase and frequency input to the local oscillator via feedback control.
As shown in FIG. 1, prior art homodyne receiver 10 includes a local oscillator or laser 12, the output of which is coupled or mixed with an optical input signal 14 via coupler 16. The coupled signal is then photo-detected and compared using conventional photo-detectors 18 and differential amplifier 20 to produce an electrical output signal 22 representative of the phase of the input signal 14. A loop filter 24 is used to electronically block unwanted noise and to electronically track phase or frequency drift, etc.
As shown in FIG. 1, prior art homodyne-type receivers require a local oscillator which must be carefully tuned to the carrier signal in order for the receiver to operate properly. Thus, a major disadvantage of conventional homodyne receivers is that they require a local oscillator and that the local oscillator must generally track the incoming signal. Thus, the oscillator linewidth requirement is very stringent. Accordingly, the local laser must maintain strict phase synchronization (provided via the above conventional phase-locked loop arrangement) for successful recovery of the received digitally phase modulated optical input signal. In order to meet these stringent requirements, an expensive laser is often used as the local oscillator having extremely narrow operating requirements. Such requirements must be maintained over varying ambient conditions (i.e., temperature, humidity, etc.).